This invention pertains to the field of electrical illuminating apparatus and particularly to such apparatus which is permissible under Mine Safety and Health Administration (MSHA) standards and regulations promulgated under the Federal Mine Safety and Health Act for use in explosive atmospheres such as coal mines.
Lighting in mines has always been relatively poor compared to working environments aboveground where minimum illumination standards for various tasks have long been established.
The difficulty of providing adequate lighting in coal mines is aggravated by the low reflectivity of the black coal and associated minerals in the roof, floor, and ribs. Rock-dusting, where employed, does provide a reflective white or light gray surface along established haulageways and heavy traffic areas such as underground maintenance shops, areas immediately adjacent the bottoms of hoisting shafts, and loading points along conveyors. These locations are generally well illuminated with permanent lighting.
By contrast, rooms where coal is actively being mined are relatively poorly lighted. They will not yet have been rock-dusted and the freshly exposed black surfaces provide no practical light reflectivity. Illumination is provided only by miners' cap lamps and one or more high intensity headlight-type lamps on each mobile mining machine. In the case of shuttle cars, which operate in both directions, there will be one or more headlights on each end. Even where such high intensity lamps are directed toward the face or toward the direction of movement of the machine, lighting is far from uniform. The operator of a continuous mining machine, or loading machine, will have enough light brilliantly illuminating the mine face to keep his machine working efficiently, but the rear boom just behind him is in relative darkness making it difficult for him to see a person immediately behind or to the side. Inasmuch as these face-working machines have conveyor discharge booms which are tiltable up and down, and swingable from side to side, there have been numerous accidents involving persons unseen by the machine operators being struck by the discharge booms and pressed against another machine or one of the side walls. Much too often, these accidents are fatal or are seriously incapacitating.
Mining Industry records show that almost all serious and fatal accidents in working places occur while self-propelled equipment is operated in them.
Pursuant to authority under the Federal Coal Mine Health and Safety Act of 1969, the Secretary of the Interior has promulgated new illumination standards for underground coal mines which, among other things specify that the entire area surrounding self-propelled mining equipment for a minimum distance of five feet be illuminated with a surface brightness of at least 0.06 footlamberts.
To provide this level of illumination, something more efficient than conventional incandescent lamps must be used. Attempts have been made to develop fluorescent lighting which is permissible for use in potentially explosive atmospheres such as coal mines, and which could provide the high level of illumination required by the new standards, but none of these have been entirely satisfactory.
One problem is that the lamps are difficult to install, involving making wiring connections to terminals at locations inside the lamp housing which are not easily accessible.
Another problem is keeping the surface temperature of the lamps, including all metal and light-transmitting components below the limits specified by Federal regulations for explosive atmospheres. Although bulbs for fluorescent lamps generally are regarded as cool to the touch, they actually have two extremely hot regions, at the cathodes adjacent the ends, where the luminous envelope locally can reach 320.degree. to 350.degree. F. Federal regulations for mine lighting prohibit surface temperatures exceeding the ambient by more than 180.degree. F. This means that in coal mines where the ambient air temperature is 60.degree. F. no part of the lamp surface in contact with the atmosphere can exceed 240.degree. F., for that specific ambient temperature.
The temperature of the light-transmitting (polycarbonate or glass) housing at the hot regions of the bulb readily exceeds the permissible limit unless the hot regions of the bulb are shielded off. This, of course, is objectionable because it blocks some of the light, reducing the efficiency of the lamp.
Still another problem is the great difference in thermal expansion coefficients of polycarbonate tubing, commonly used for the light-transmitting housing, and steel and brass, commonly used for the other parts of the lamp. A typical polycarbonate composition and typical steel and brass compositions have thermal expansion coefficients as follows:
Polycarbonate--6.6.times.10.sup.-5 in./in./deg.C.
Brass--1.9.times.10.sup.-5 in./in./deg.C.
Steel--1.05.times.10.sup.-5 in./in./deg.C.
Thus, polycarbonate expands and contracts roughly 31/2 times as fast as brass, and 6 times as fast as steel, with changing temperatures. This poses a serious problem during heating and cooling cycles at common interfaces between polycarbonate and either of the two metals. Differential expansion and contraction can crack the polycarbonate material and make the lamp hazardous in explosive atmospheres.